In recent years, four-wheel vehicles (rear wheel toe angle variable vehicles) have been developed that have a rear wheel toe angle control mechanism in addition to a front wheel steering mechanism, the rear wheel toe angle control mechanism being configured to variably control the toe angles of the left and right rear wheels according to a front wheel steering amount and a vehicle speed, and the front wheel steering mechanism being configured to steer the left and right front wheels according to a rotational operation of a steering wheel by a driver. In a rear wheel toe angle variable vehicle, the left and right rear wheels are steered in-phase or out-phase (toe-in or toe-out) according to an amount of steering operation by a driver and motion state parameters of a vehicle (a vehicle speed, a yaw rate, a lateral acceleration, a longitudinal acceleration). The rear wheel toe angle control mechanism includes a king pin which pivotably supports a knuckle and a hub carrier (hereinafter collectively referred to as a knuckle), and a rear toe actuator which connects the vehicle body to the knuckle. The rear toe actuator is drive-controlled by a rear wheel steering ECU (Electronic Control Unit) (see Japanese Unexamined Patent Application Publication No. 2011-131859).
The rear toe actuator disclosed in Japanese Unexamined Patent Application Publication No. 2011-131859 includes a housing connected to a vehicle body; a rod which is slidably held in the housing with one end of the rod connected to the knuckle; a nut fixed to the inside of the rod; a screw shaft which is rotatably held in the housing and screwed into the nut (that is to say, the screw shaft and the nut constitute a feed screw); a motor which drives the screw shaft via a deceleration mechanism; and a stroke sensor which detects an axial displacement of the rod with respect to the housing. A signal detected by the stroke sensor is outputted to the rear wheel steering ECU which detects (estimates) the actual toe angle based on the detected signal, and performs feedback control on the motor of the rear toe actuator so as to reduce the difference between a target toe angle and the actual toe angle.